How to Stop iCloud from Using Cellular Data for Transfers on an iPhone

Open your iPhone's Settings. Do so by tapping the grey gear icon on one of your Home screens (it might also be in a folder named "Utilities").

Scroll to the fourth group of options and select iCloud.

Tap iCloud Drive. If your iCloud Drive is disabled, your data isn't being used for transfers.

Scroll to the bottom of the page.

Slide the Use Cellular Data switch left to the "Off" position. It should turn grey, signifying that iCloud will only transfer your documents and data when your phone is connected to wifi.